1. Field of the Invention
The present invention relates to a communication system, and more particularly, to a method and apparatus for progressively scanning channels.
2. Description of the Related Art
Wireless local area network (WLAN) technology is now popularly used in various applications. Numerous organizations devote extensive resources to research, seeking improvements in WLAN data communication quality. In a WLAN, a wireless transmit/receive unit (WTRU) may be located within communication ranges of several access points (APs). However, the AP associated with the WTRU and the channel the WTRU operates on may change over time due to signal strength variations. Before the WTRU can be associated with an AP, a handoff procedure has to be performed. The handoff procedure includes a scanning phase, an authentication phase and a re-association phase, with the scanning phase incurring significant delay. For example, a dual-band station which could operate on 38 different channels (channels 1-14, channel 36, channel 40, channel 44, channel 48, channel 52, channel 56, channel 60, channel 64, channel 100, channel 104, channel 108, channel 112, channel 116, channel 120, channel 124, channel 128, channel 132, channel 136, channel 140, channel 149, channel 153, channel 157, channel 161, channel 165) that requires 140 ms to scan each channel. Such a dual-band station would need 5320 ms to scan all the channels. However, if the operating system used for the dual-band station is Microsoft Windows XP, after running a channel scanning procedure for approximately 3 seconds, the operating system interrupts the scanning procedure automatically. Thus, the dual-band station can not complete the procedure of scanning all channels. The dual-band station is unable to connect with an AP if the channel the AP operates on is not scanned by the dual-band station.
FIG. 1 illustrates a diagram for a scanning procedure performed by a station. An operating system issues a scan request to a wireless device driver of a station. The wireless device driver deletes all previous channel scanning results and then starts scanning from channel 1. As above, after running the channel scanning procedure for approximately 3 seconds, the operating system interrupts the scanning procedure automatically and issues a listing request. Therefore, the scanning procedure can only complete the scanning of channels 1 to 64. Because the wireless device driver deletes all previous channel scanning results before beginning a scanning operation and then starts scanning from channel 1, only some channels, channels 1 to 64 in this case, are scanned for every single channel scanning procedure. In other words, the station is unable to connect to an AP operating on another channel (for example, channel 153) which is not scanned yet.